This invention relates generally to methods and apparatus for optimizing or at least improving volume renderings in images. The methods and apparatus are particularly useful in medical imaging.
Visualization by volume rendering is a well-known technique to produce realistic images of volumetric objects. One method of producing volume renderings is to cast rays through the volume and record the “ray values” when the rays pass a plane behind the object. The “ray value” recorded in the view plane is a combination of the values of all the voxels along the path from the viewpoint to the view plane. Typically, the combination is the sum of the voxel value, each multiplied by a certain weighting value called “opacity”.
Cardiac ultrasound data is of highly varying image quality (IQ). Even in high IQ datasets, there is heterogeneous gray-scale intensity along chamber boundaries. Furthermore, the gray-scale intensity often changes during the cardiac cycle. This makes it very challenging to generate high quality volume renderings. The current solution is to make a “soft” opacity function giving a fuzzy definition of an object boundary. However, what the user then perceives as an inner wall of a chamber might not be the inner wall.
Automated segmentation methods are commonly used when measuring object volumes in volumetric image data. Various types of three-dimensional (3D) segmentation algorithms have been developed. Most algorithms have in common that an elastic model deforms towards edges in the volumetric image data.
Shell rendering is an alternative to traditional ray-casting based volume renderings. Shell renderings overcome storage and speed issues of ray casting techniques. A shell consists of a set of voxels near the object boundary together with a number of attributes associated with the voxels in this shell. If there is low confidence in the object boundaries, voxels far from the actual boundary may also belong to the shell. Increased rendering speed is achieved by projecting only the voxels within the shell on to the view plane (voxel projection). A boundary-likelihood may be assigned to each voxel to allow measurement of distances directly in the volume rendering. As an example of a boundary likelihood function, a normalized magnitude of the image gradient may be used. The choice of the boundary-likelihood function must relate to the opacity to make the rendering relate intuitively to what is being measured.
Methods for combining ray casting with segmentation are also known. The methods utilize an “object mask” file that contains information about to which object each voxel belongs. A “superficial rendering” based on projecting voxels in the object boundary vicinity on to the view plane and a “deep rendering” based on maximum intensity projection of all voxels within the object are both provided. Using this approach, the actual image data is displayed and allows the user to “turn off” any obscuring object. The segmentation boundary also may be dilated so that the surrounding image data can be seen if the segmentation results are in doubt.
A method using a fully automatic endocardial segmentation technique to improve volume renderings of ultrasound data is also known. The technique includes a method of voxel opacity assignment based on the voxel location relative to the segmented endocardium (the Euclidian distance from the segmented boundary modulated the opacity function) and the voxel intensity after applying an anisotropic filter. In this approach, the opacity function is partly controlled by the segmentation. However, the opacity function is not adapted to the intensity of the boundary data.
Methods for global and regional optimization of the opacity function when rendering ultrasound data are also known. In these methods, the projected rays of the volume rendering are analyzed to improve the opacity function. These approaches use a local edge detector and not a global segmentation algorithm in order to estimate the opacity function(s).
The known approaches and techniques do not use the combination of a global segmentation method and regional opacity functions to improve the volume rendering. Accordingly, image quality of the volume rendering may be degraded.